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Cycle Skip: Definition, Sonic Log Error, and Quality Control

What Is Cycle Skip?

Cycle skip is a false high transit-time reading on a sonic or acoustic log. It happens when the logging tool misses the first usable sound-wave arrival and locks onto a later wave cycle. EPA acoustic-logging guidance describes cycle skipping as a common acoustic-log problem caused by weak first arrivals or strong early noise.

Key Takeaways

  • Cycle skip is usually a measurement-picking error, not a sudden real change in the rock.
  • The log reads too slow because the tool counted a later acoustic cycle as the first arrival.
  • Gas can cause cycle skip, but fractures, washout, unconsolidated rock, aerated mud, and poor tool position can also do it.
  • Bad cycle-skip editing can damage porosity, synthetic seismograms, geomechanics, and velocity models.
  • The raw waveform, caliper, density, neutron, image log, and mud log should be checked before accepting or deleting the spike.

How Cycle Skip Happens

A sonic tool is listening for a tiny event in a noisy place. It sends an acoustic pulse, waits for that pulse to travel through the borehole and formation, then marks the arrival time. In clean hole with strong rock response, the first arrival is obvious. In messy hole, the first arrival may be weak, broken, or hidden inside noise.

Cycle skip is what happens when the tool picks the wrong moment. Imagine clapping in a canyon and counting the second echo as if it were the first. The echo is real, but the interpretation is wrong. On the log, that mistake becomes a sharp high transit-time spike, making the rock look slower than it really is.

Why It Is Not Always Gas

Field crews often associate cycle skip with gas because gas weakens acoustic arrivals. That association is useful, but incomplete. EPA acoustic-logging material makes the broader point: anything that weakens the first compressional arrival or creates strong pre-arrival noise can cause cycle skipping. A washed-out hole, a fractured carbonate, poor centralization, or aerated drilling mud can all create the same visual symptom.

That is why the log should not be edited like a spreadsheet typo. A spike beside a caliper washout may be borehole damage. A spike that lines up with image-log fractures may still be telling a real story about broken rock. A spike with no supporting evidence is more likely a tool-picking problem.

How to Read Cycle Skip in Context

The useful mental habit is to separate the measurement from the formation. A sonic log is not a photograph of rock. It is a chain of events: transmitter fires, wave travels, receiver listens, software picks an arrival, and the curve is drawn. Cycle skip lives in that chain. Once the reader sees the chain, the quality-control question becomes obvious: which link failed?

Fast Facts

  • Cycle-skip spikes often jump by about one waveform cycle.
  • That jump is why the curve can look sudden and mechanical rather than geological.
  • A spike still deserves context: waveform, caliper, mud log, and image evidence decide whether the interval is bad data or bad hole.

Tip: Do not smooth sonic spikes first and ask questions later. Look at the waveform and the caliper before deciding whether the curve or the rock is lying.

Cycle Skip is also known as:

  • cycle skipping: common log-quality wording.
  • missed cycle: plain description of the error.
  • sonic spike: field description of how it appears.

Related terms: sonic log, wireline log, caliper log.

Frequently Asked Questions

What causes cycle skip?

A weak or noisy acoustic signal makes the receiver mark the wrong wave cycle. Gas, fractures, washout, tool position, and poor borehole conditions can all contribute.

Is cycle skip always bad data?

It is a warning sign. The picked arrival may be wrong, but the interval may still mark a real borehole or formation problem.

How is cycle skip corrected?

Interpreters review waveforms and supporting logs, flag the bad interval, and edit only where the evidence supports correction.

Why Cycle Skip Matters in Oil and Gas

Cycle Skip matters because it connects a word to a real decision in the field, the lab, or the interpretation room. A useful definition should make the concept clear enough to act on, technical enough to avoid false confidence, and specific enough that the reader understands what can go wrong. That is the standard for this glossary: plain language first, evidence underneath, and enough operational context that the term feels connected to actual oil and gas work. The article should teach, not merely label. If a reader leaves knowing what to check next, the page has done its job.

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